1. Field of the Invention
The present invention relates to a servo controller and a control method using the servo controller, and, in particular, to a servo controller which is used to assist steering operation of a vehicle, and a control method using the servo controller.
2. Description of the Related Art
In the field of servo control, technologies using integrators to eliminate the steady-state error caused by disturbance have been proposed (see Japanese Patent Application Publication No. 2001-1923, for example). The technology described in Japanese Patent Application Publication No. 2001-1923 relates to a vehicular steering assist system, which is an example of the servo control. In this document, the precision of lane keeping control, for example, is improved by calculating, by integral calculation, a value representing the influence exerted on a vehicle due to disturbance, and controlling the steering assist force of the vehicle according to the integral value.
However, if a process for eliminating the steady-state error caused by disturbance is performed using an integrator in this way, a phase delay due to the characteristics intrinsic to integral calculation is inevitably caused. Such a phase delay can result in a control result lacking in stability, depending on the behavior of the disturbance. For example, there is a possibility that, when there is no disturbance, low frequency oscillations occur in controlled variables. If there is a disturbance, and the direction thereof is inverted, the integral value which is calculated to obtain a correct control result may increase an influence of the disturbance.
For the purpose of solving such problems, it is possible to adopt a method used to achieve stable control by resetting an integral value of a parameter to zero when the direction of the slope of the parameter to be integrated is inverted. With this method, since quick response can be made even when there is a disturbance and the direction thereof is inverted, it is possible to maintain the stability of control. When a steady disturbance is being caused, however, oscillations occur in controlled variables, and, therefore, there is a problem that the stability of control cannot be maintained.
It is also possible to adopt a method used to achieve stable control by resetting an integral value of a parameter to zero when the sign of the parameter to be integrated is inverted. With this method, since the steady-state error caused by disturbance is stably eliminated by an integrator even when steady disturbances are being caused, it is possible to achieve a good control result. There still remains a problem, however, that, when the direction of the disturbance is inverted, the time from when the parameter is reset to when a stable control is achieved, is excessively long, and the disturbance cannot be dealt with during the transition period.